\documentclass{article}

\usepackage[export]{adjustbox}
\usepackage{booktabs}
\usepackage{calc}
\usepackage{rotating}
\usepackage{multirow}
\usepackage[margin=0.1cm, hoffset=-0.5cm, paperwidth=18cm, paperheight=23.1cm]{geometry}

\begin{document}
\makebox[1\textwidth]{%\parbox{\dimexpr\linewidth-0.5\fboxsep-0.5\fboxrule}{ 
											\parbox{\textwidth}{
											\centering
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_UnionB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_PublicSectorB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_ManufacturingSectorB.png}\\
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_PartTimeB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_NonWhiteB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_FemaleB.png}\\
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_MarriedB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_UrbanB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_HighSchoolB.png}\\
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_SomeCollegeB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_AssociateDegreeB.png}
											\adjincludegraphics[width=0.32\textwidth,trim={0.4cm 0.4cm 0.4cm 0.4cm},clip]{QR_Coef_CollegeB.png}\\
											}
}
\end{document}